The crater, nicknamed 'The Door to Hell' is about 69 metres wide and 30 metres deep. It is located in a natural gas field in Ahal Province in Turkmenistan.
The Door to Hell is noted for its natural gas fire which has been burning continuously since it was lit by Soviet petroleum engineers in 1971. The Soviet Engineers set it on fire when they discovered poisonous gas was emitting from the place. They expected the gas will be exhausted in a week after the gas must have finished, but over 40 years after the fire is still burning.
The crater is now a minor tourist attraction in Turkmenistan.

Links:
http://www.theguardian.com/world/2014/jul/18/-sp-door-to-hell-turkmenistan-in-pictures
http://en.wikipedia.org/wiki/Door_to_Hell